Fixing add button on linked resources widget in incidents

What does this MR do and why?

Will theoretically fix #372484 (closed), which aims to make the make the add button on the Linked resources widget on Incidents match the rest of the Linked widgets, which look like this:

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Enable FF --> Feature.enable(:incident_resource_links_widget)
2. Go to Monitor > Incidents and create a new incident.
3. View the newly created incident. The Add button on the Linked resources button should now match the Add button on the Tasks and Linked Items widgets.

MR acceptance checklist

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